The present invention relates to a self-propelled working machine, especially in the form of a surface milling machine, such as for example a Surface Miner, asphalt-milling machine or snow-milling machine, with a main working and/or power unit which can be operated in a steady-state or nearly steady-state operating status and which can be driven by a drive device comprising at least one electrical motor, the electrical motor being equipped with a start-up circuit including a frequency converter to limit starting current. The invention also relates to a process for operating such a self-propelled working machine.
Surface Miners are for example continuously operating surface mining machines using a rotating roll for grinding rocks or soil in a milling action and which usually continuously advance by means of caterpillars in order to force the roll into the rock. In this approach said roll constitutes the main operating unit which requires high energy input and thus also requiring a suitable drive system. In this regard, DE 10 2007 007 996 B4 discloses a diesel electrical drive system wherein the mill roll of the Surface Miner is driven by an electrical motor supplied with electrical power by a generator, which in turn is driven by a diesel engine. Further embodiments of Surface Miners are disclosed in the references WO 03/058031 A1, DE 10 2008 008 260 A1, DE 10 2007 044 090 A1, DE 10 2007 028 812 B4, DE 199 41 800 C2, DE 199 41 799 C2 or DE 20 2007 002 403 U1.
Such electrical drives have considerable advantages over hydrostatic drive systems, such as especially higher efficiency and greater ease of maintenance. Due to substantially better efficiency resulting in lower operational costs, wherein the latter being quite remarkable in regard of required engine performance, higher costs of purchase for electrical motors may be compensated in a reasonably short period of time. The concept of a comparable diesel electrical drive hence lends itself not only for the use in Surface Miners but also in similar self-propelled working machines, such as asphalt milling machines, snow milling machines, or also in agricultural machines such as combined harvesters or the like which during processing work continuously and near steady-state, i.e. performing an especially rotative main working motion at constant or nearly constant rpm, respectively, and wherein the driving motion represents the feed motion. In this context, “steady-state” operational status does not necessarily mean “exactly constant” in the sense that said main operating unit is actually operated at exactly constant rpm but also includes minor variations near the operational set point, for example due to variations of rpm of the diesel engine.
However, start-up procedure of such diesel electrical drive systems for said type of processing machines poses problems. Direct starting the drive motor connected to the generator is not useful since in this case a very high start-up current will occur which can be five or six times that of the nominal current and for which the entire system would have to be suitably dimensioned or overdimensioned respectively.
Therefore the use of gentle start-up circuits wherein start-up current is limited by lowering voltage is well known. However, this is only possible if no or almost no torque is required during start-up. If only a starting torque is required that is smaller than one third of the motor's starting torque in case of direct starting, the working motor may also be started by means of a star-delta-connection. However, even in this approach high start-up current still occurs which is generally significantly higher than nominal current and must be taken into account when dimensioning the generator, resulting in that the latter becoming bigger and more expensive.
However, in the case of Surface Miners a rather high starting torque might be required, for example for jerkily loosening a mill roll after it became frozen. In cases wherein an essential part of the nominal torque or even a higher starting torque which might be twice as high as the nominal torque is required due to an external load torque or required start-up times installation of a frequency converter is known converting the frequency supplied by the generator in order to limit incoming current during start-up. As shown in FIG. 10, the respective frequency converter FU is inserted between the generator G and the electrical motor M of the drive system.
Although the (main) work drive system while in use is operated at constant or almost constant rpm, respectively, feeding then will take place via said frequency converter during the entire period of operation, thus requiring the frequency converter to be at least dimensioned according to the nominal power of the work drive system. This is disadvantageous with respect to losses, efficiency, operational costs and wear.